Driving circuits for electrical printers

ABSTRACT

A driving circuit for use in electrical printers has at least one driving transistor to the base of which is a control pulse is applies. A solenoid is connected to the emitter of the driving transistor for operating a mechanical print driving member when the solenoid is energized by an exciting pulse which is produced at a condenser whose terminal voltage is kept at a voltage higher than that of the exciting pulse.

United States Patent 11 1 Tanabe 1 Aug. 19, 1975 DRIVING CIRCUITS FOR ELECTRICAL PRINTERS [75] Inventor: Koji Tanabe, Higashi-murayama,

Japan Citizen Watch Co., Ltd., Tokyo, Japan Filed: May 31, 1973 Appl. No.: 365,589

[73] Assignee:

[30] Foreign Application Priority Data June 6, 1972 Japan 47-66539 U.S. Cl. 197/1 R; 317/1485 R Int. Cl B41j 3/05 Field 01 Search 197/1 R; 101/93 C;

317/1485, 137, DIG. 4, DIG. 6

References Cited UNITED STATES PATENTS 1/1967 Burley 317/137 3,432,844 3/1969 Winston 197/1 R 3,624,661 11/1971 Shebanow et 197/1 UX 3,638,197 1/1972 Brennan et a1 197/1 R X 3,705,333 12/1972 Galetto et a1. 317/1485 R 3,752,288 8/1973 Dejig et a1 197/1 R 3,770,092 11/1973 Grim 197/1 R Primary Examiner-Edgar S. Burr Assistant ExaminerR. T. Rader Attorney, Agent, or FirmErnest G. Montague; Karl F. Ross; Herbert Dubno ABSTRACT 5 Claims, 6 Drawing Figures I EHAMMER PIP/IV 7' -HAMME R SOLENOID PATENTED AUEI 9I975 FIG. 2a

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I PRINT-HAMMER sow/vow PATENTEDAUE 1 9197s SHEET 2 [IF 2 DRIVING CIRCUITS FOR ELECTRICAL PRINTERS FIELD OF THE INVENTION My invention relates to a driving circuit for use in electrical printers and more particularly to a driving circuit which is capable of being operated by a control pulse and can excite a solenoid for operating a mechanical print driving-member, for example, a print drivinghammer etc. with the aid of an exciting pulse produced across a condenser.

BACKGROUND OF THE INVENTION In general, there are various kinds of electrical printers such as a hammer-driving type, a needle-dot type, a heat sensitive print type, an ink jet type electrical printer, etc. The driving circuit according to my invention is particularly useful for the hammer-driving type or needle-dot type electrical printer in which the solenoid for operating the mechanical pring drivingmember is energized by an exciting pulse produced from a condenser. As a result, the exciting pulse applied to the solenoid has a voltage waveform which is uniform in level.

An application of an exciting pulse having a voltage waveform of uneven level may result in a change of the electromagnetic force produced by the solenoid, thereby changing the mechanical force urged against the print-driving member and hence changing the printing speed. As a result, the printing characteristics of the electrical printer may become degraded.

OBJECTS OF THE INVENTION An object of my invention is to provide a driving circuit which can obviate disadvantages encountered by the conventional driving-circuits heretofore and described in further detail in conjunction with the specific description of my improved driving circuit; which is simple in construction and reliable in operation and hence is particularly useful for electrical printers.

Another object of my invention is to provide a driving circuit which is not influenced by any voltage variation across a current supply-circuit and hence does not require any special current supplycircuit.

A further object of my invention is to provide a driving circuit which is capable of using a current supplycircuit having a small current-carrying capacity and obtaining significantly good printing characteristics.

SUMMARY OF THE INVENTION My invention provides a driving circuit for use in electrical printers comprising a control-pulse supplycircuit, at least one driving transistor whose base is connected to the control-pulse supply-circuit, a solenoid connected to the emitter of the driving transistor and adapted to operate a mechanical print driving-member when energizing, a condenser connected between the collector of the driving transistor and the solenoid for generating an exciting pulse for energizing the solenoid, and a current supply-circuit connected in parallel with the condenser, the terminal voltage of the condenser being kept at a voltage higher than the voltage of the exciting pulse.

BRIEF DESCRIPTION OF THE DRAWING Other features and advantages of my invention will becomme apparent from the following description in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram of a conventional driving circuit for use in electrical printers;

FIG. 2a shows a voltage wave-form diagram of a conventional driving circuit shown in FIG. 1 for illustrating a varying exciting pulse voltage-level;

FIG. 2b shows a voltage wave form diagram of a driving circuit for use in electrical printers according to my invention for illustrating a uniform exciting-pulse voltage level;

FIG. 3 is a schematic circuit diagram of a first embodiment of a driving circuit for use in electrical printers according to my invention;

FIG. 4 is a schematic circuit diagram of a second embodiment of a driving circuit for use in electrical printers according to my invention; and

FIG. 5 is a schematic circuit diagram of a third embodiment of a driving circuit for use in electrical printers according to my invention.

SPECIFIC DESCRIPTION A conventional driving circuit of prior art for use in electrical printers as shown in FIG. 1 includes a driving transistor 1 whose collector electrode is connected to one end of a solenoid 2, the other end of which is connected through an electric current supply circuit 3 to the emitter electrode of driving transistor 1. Across the base and emitter electrodes of driving transistor 1 is applied a control pulse 4 to make driving transistor 1 conductive. Thus, a current amplification of the control pulse 4 is effected by means of driving transistor 1. The solenoid 2 is energized by an exciting pulse suppled from the electric current supply source 3 to operate, for example, a driving hammer (not shown) such that the driving hammer is urged against a print wheel (not shown), thereby completing a desired printing operation.

As stated hereinbefore, a conventional driving circuit uses a driving transistor 1 whose collector is connected to the solenoid 2 so that the driving circuit cannot be applied to the electrical printer to operate it reliably. That is, in the conventional driving circuit shown in FIG. 1 the control pulse 4 merely serves to make driving transistor 1 conductive while the exciting pulse supplied to solenoid 2 is controlled by the characteristics of the electric current supply circuit 3. As a result, a variation in voltage of the electric current supply circuit 3 directly results in a voltage wave variation of the exciting pulse applied to solenoid 2 thereby degrading the characteristics of the electric printer. In order to obviate such a disadvantage, certain special precautions must be taken for stabilizing the electric current supply circuit 3, rendering, the apparatus as a whole large in size and expensive. In electrical printers, a plurality of solenoids 2 must simultaneously be excited for the purpose of printing a plurality of figures, and as a result, a special electric current supply circuit 3 having a large current capacity is required.

Heretofore, it has been proposed to incorporate a condenser, for example condenser 3', into the electric current supply circuit 3 for the purpose of exciting a plurality of solenoids 2. This construction makes it possible to charge condenser 3' overa relatively long period of time and 'at the end of this period to discharge the condenser and hence supply the discharging current to the solenoids 2 whereby the printing operation is completed. Thus, it is possible-to decrease the current-carrying capacity instantaneously required for the electric current-supply circuit 3 per se. In that case, however, the discharge current supplied to solenoids 2 is substantially determined by the discharging characteristicsof the'condenser and its associated circuit, and as a result, it is not possible to obtain exciting pulses having a uniform voltage level.

FIG. 2a shows the variation of exciting pulse voltage V; across the condenser 3 as a function of time 2 for the above mentioned conventional driving circuit for use in electrical printers. At time t a control pulse 4 is applied to the base of driving transistor 1 to make transistor 1 conductive. Thus, the terminal voltage V across the condenser is applied to solenoid 2. The con denser is discharged up to a time to produce an exciting pulse which is substantially triangular in shape as shown in FIGf2a depending on the discharging characteristics of the condenser and its associated circuit. From t at which time control pulse 4 ceases condenser 3 is charged again until time t as shown by a dotted line. From time t;, at which time control pule 4 reappears, condenser 3 is again discharged up to time t.,,

ingcircuit provides exciting pulse voltages whose waveformsaredetermined by the discharging characteristics of the condenser and its associated circuit. If the time constant of the discharging circuit is small, the waveform of the exciting pulse voltage becomes substantially triangular in shape as shown in FIG. 2a. Such a triangular voltage waveform of the exciting pulse cannot produce a driving force which can operate the electrical printer reliably.

In order to obviate that disadvantage it is necessary to increase the current-carrying capacity of condenser 3' making the apparatus unduly large.

If solenoid 2 is energized after a short interval of time following 1 for example, at t as shown in FIG. 2a, it is impossible to charge the condenser sufficiently, and as a result, only a low terminal voltage V of the condenser shown in FIG. 2a is supplied to solenoid 2, the driving potential getting correspondingly decreased.

In FIG. 3 is shown a first embodiment of my invention in which P P P -designate a plurality of driving circuits which are identical in construction and correspond to respective characters of an electrical printer. Each driving circuit comprises first and second driving transistors 5 and 6 respectively of substantially similar characteristics and connected in the conventional, Darlington fashion, elements of the second and third driving circuits shown being denoted by respective suffixes out e.g. 5 and 5". A control signal 7 from a control pulse supply circuit 7' is supplied to the base of the first driving transistor 5. To the emitter of a second driving transistor 6 there is connected one terminalof a solenoid 8 slanted by a diode 9. A conventional electric current-supply circuit consists of a stepdown transformer 10 for lowering the voltage of a conventional alternating frequency electricsupply source and of a full'wave rectifier circuit 11. The output terminals of full wave rectifier circuit 11 are connected in parallel with a condenseer 12 and with the above mentioned driving circuits P P through P The driving circuit shown in FIG. 3 operates as follows.

In the absence of a control pulse 7, the first and second driving transistors 5 and 6 associated with a character of the printer are non-conductive; at this time condenser 12 is sufficiently charged from electric current supply source S. The period of time for charging condenser 12 is determined by the time constant of the charging circuit, and a resistor may be added for suitably increasing that time constant. In addition, the charging time for condenser 12 may be suitably determined as a function of the space between successive character print-outs of the electrical printer. Thus, it is possible to determine the optimum charging time of condenser 12 for the highest printing speed. i

If control pulse 7 is applied to the base of the first driving transistor 5 of the driving circuit associated with a predetermined character, both first and second driving transistors 5 and 6 become conductive to complete a conductive path from condenser 12 through second driving transistor 6 to solenoid 8. This allows a discharge current from charged condenser 12 to be supplied to solenoid 8 energizing the latter, and as a re--' sult, energized solenoid 8 urges a driving hammer (not shown) against a print wheel (not shown) or projects a print needle onto a print completing a desired character imprint.

In the absence of control signal 7, both first and second driving transistors 5 and 6 are nonconductive and solenoid 8 is deenergized. A counter electromotiveforce produced at this instant as a result of cessation of current through solenoid 8 is eliminated by a diode 9 in the well known manner.

The above mentioned operation will now be described in detail with reference to the waveform diagram shown in FIG. 2b in which the exciting pulsevoltages applied to solenoid 8 are shown by a full line and the terminal voltage across condenser 12 is shown by dotted lines.

At time t condenser 12 is charged to a saturation terminal voltage V and if control pulse 7 is applied to the base of the first driving transistor 5 the exciting pulse-voltage V is applied to solenoid 8. In accordance with the invention solenoid 8 is connected to the emitter of the second driving transistor 6 so that the saturation terminal-voltage V of condenser 12 is not applied directly to solenoid 8. The exciting pulse voltage V applied to solenoid 8 is substantially determined by control pulse 7, i.e., the value and waveform of the exciting pulse voltage V., are a function of control pulse 7. At time t the terminal voltage of the condenser 12 decreases to V but the same exciting pulse voltage V, is maintained during the period from time t to t across solenoid 8 thereby exciting the latter with a voltage pulse of uniform level.

Experimental tests have shown that aa driving circuit of extremely high energy efficiency is obtained if the saturation terminal-voltage V of the condenser 12 is substantially twice that of exciting pulse-voltage V As shown in FIG. 2b, if the solenoid 8 is deenergized at time t a counter electromotive-force v shown dotted is produced. This counter electromotive-force v may be suppressed to a value v which is determined by the forward resistance value of diode 9 and is considerably lower than the breakdown voltage of first and second driving transistors 5 and 6.

If control pulse 7 is not applied to the base of the first driving transistor 5 during the time period to condenser 12 is again charged from electric-current supply source S to a terminal voltage V as shown by a dotted line. If at time t the control pulse 7 is applied to the base of the first driving-transistor 5, solenoid 8 is again energized for the time period t to t.,, the above sequence of events being thereby repeated.

As stated hereinbefore, my invention makes it possible to excite solenoid 8 independently of the terminal voltage across condenser 12. As a result, if control pulse 7 is applied to the base of first driving transistor 5 after a small interval of time, e.g. from t, to t condenser 12 is on longer charged to its saturation terminal-voltage V but is instead charged to a relatively lower terminal voltage as shown in FIG. 2b. This has no influence upon exciting pulse voltage V In FIG. 4 is shown a second embodiment of the invention having elements substantially similar to those of the first embodiment, such elements being denoted by the same reference numerals.

In the second embodiment of my invention shown in FIG. 4, a resistor 13 is connected between the base of the first driving transistor 5 and in lieu of solenoid 8 diode 9 shown in FIG. 3. In this embodiment, resistor 13 considerably reduces the electromotive-force produced in solenoid 8. That is, if the counter electromotive force is produced in solenoid 8 to decrease the emitter voltage of the second driving transistor 6, a weak current flows from solenoid 8 through resistor 13 first and second driving transistors 5 and 6 reentering second driving transistor 6 conductive. As a result, so-

7 lenoid 8 is supplied with a current counteracting the electromotive force produced therein. Thus, this embodiment is substantially capable of rapidly and reliably eliminating a counter electromotive-force original in within solenoid 8.

In the second embodiment of my invention, provision may be made for an emitter follower-type transistor connected to the base of the first driving transistor 5 to amplify the weak current of control pulse 7. In this case, resistor 13 may be used as a load resistor for such an emitter-follower transistor.

FIG. 5 shows a third embodiment of the driving circuit according to my invention in which each control pulse 7 is first amplified by a preamplifier circuit 14 and then supplied to a clamping circuit 15 whose output is applied to the base of each driving transistor 5 of respective driving circuits P. of each driving transistor 5 and energized by the exciting pulse in the same manner as explained with reference to the first and second embodiments of my invention.

In the third embodiment of my invention the voltage level V. of the exciting pulse may be adjusted by merely changing the level of clamping circuit 15, by, for example altering the value of resistor and so controlling the maximum positive voltage fed and for example, to the base of transistor 5, whereby the print can be operated in a reliable fashion.

In the first, second and third embodiments of my invention shown in FIGS. 3, 4 and 5 respectively, use is made of a condenser 12 common to all of driving circuits P I P,,.

The same result may also be obtained even when condenser 12 is separately associated with each of the driving circuits P P P As stated hereinbefore, the driving circuit for use in electrical printers according to my invention has a number of advantages. In the first place a solenoid is connected to an emitter of a driving transistor and is energized by a discharge current from a condenser so that it is possible to make the voltage level of the exciting pulse independent of the voltage variation in an electric current supply-circuit. Secondly, the voltage level of the exciting pulse is always uniform so that the printing characteristics of electrical printers and more particularly of a high speed electrical printer may be improved. Third, if a control pulse is generated from an integrated circuit etc. whose output voltage waveform level is extremely uniform, the solenoid may be energized by an exciting pulse whose voltage level is correspondingly uniform. Fourth, a special current supplycircuit is not required and hence use may be made of a current supply-circuit having a relatively small current-carrying capacity. Finally any, electromotive force produced in the solenoid may be substantially eliminated in a simple and reliable manner.

I claim:

1. A driving circuit for an electrical printer comprising:

a control-pulse supply circuit;

at least one driving transistor having an emitter electrode, a collector electrode and a base electrode, said base electrode being connected to said control-pulse supply circuit;

a solenoid having one terminal connected to the emitter electrode of said driving transistor and adapted to operate a mechanical print-driving member;

a capacitor connected directly to the collector electrode of said driving transistor and between said collector electrode and another terminal of said solenoid for generating an exciting pulse for energizing said solenoid and operating said mechanical print-driving member; current-supply circuit shunted across said capacitor, the terminalvoltage of said capacitor being at a voltage higher than that of said exciting pulse; and

a two-terminal shunting element having a first terminal connected to said other terminal of said solenoid, and a second terminal element returned to said driving transistor for eliminating a counterelectromotive force produced in said solenoid upon de-energization thereof by cessation of said exciting pulse.

2. A driving circuit as defined in claim 1 wherein a plurality of driving circuits corresponding to respective characters of an electrical printer are connected in parallel with said capacitor and said current-supply circuit, a control pulse being supplied to the base of the driving transistor of each respective driving circuit.

3. A driving circuit as defined in claim 1 wherein said element is a diode connected between said second terminal and said emitter electrode.

4. A driving circuit as defined in claim 1 wherein said element is a resistor connected between said base electrode of said driving transistor and said solenoid.

5. A driving circuit as defined in claim 1 wherein said control-pulse supply circuit comprises:

a preamplifier circuit amplifying said control pulse and having an output applied to the base of said driving transistor of said driving circuit; and

a clamping circuit for maintaining substantially constant the level of said exciting pulse. 

1. A driving circuit for an electrical printer comprising: a control-pulse supply circuit; at least one driving transistor having an emitter electrode, a collector electrode and a base electrode, said base electrode being connected to said control-pulse supply circuit; a solenoid having one terminal connected to the emitter electrode of said driving transistor and adapted to operate a mechanical print-driving member; a caPacitor connected directly to the collector electrode of said driving transistor and between said collector electrode and another terminal of said solenoid for generating an exciting pulse for energizing said solenoid and operating said mechanical print-driving member; a current-supply circuit shunted across said capacitor, the terminal voltage of said capacitor being at a voltage higher than that of said exciting pulse; and a two-terminal shunting element having a first terminal connected to said other terminal of said solenoid, and a second terminal element returned to said driving transistor for eliminating a counter-electromotive force produced in said solenoid upon de-energization thereof by cessation of said exciting pulse.
 2. A driving circuit as defined in claim 1 wherein a plurality of driving circuits corresponding to respective characters of an electrical printer are connected in parallel with said capacitor and said current-supply circuit, a control pulse being supplied to the base of the driving transistor of each respective driving circuit.
 3. A driving circuit as defined in claim 1 wherein said element is a diode connected between said second terminal and said emitter electrode.
 4. A driving circuit as defined in claim 1 wherein said element is a resistor connected between said base electrode of said driving transistor and said solenoid.
 5. A driving circuit as defined in claim 1 wherein said control-pulse supply circuit comprises: a preamplifier circuit amplifying said control pulse and having an output applied to the base of said driving transistor of said driving circuit; and a clamping circuit for maintaining substantially constant the level of said exciting pulse. 